The present invention relates to mirrors and particularly to mirrors that are formed utilizing flat substrates such as glass which subsequently are heat-bent into a desired curved configuration.
Curved mirrors commonly are employed as rear-view mirrors for motor vehicles, as reflecting surfaces for telescopes and the like. They may be formed by first bending a sheet of glass into a desired curved configuration, and subsequently apply a reflecting coating to one side or the other of the substrate. For example, curved mirrors of the type used in carnivals to provide amusing, contorted reflections of a viewer may be made by first forming a sheet into the desired shape, and then coating one surface of the glass with metallic silver and a protective paint overcoat.
Mirrors also can be manufactured by applying a reflective coating to a glass substrate using a magnetron sputtering technique of the type described in Chapin, U.S. Pat. No. 4,166,018. Chromium or silver may be employed as the reflective layer. When curved mirrors are manufactured using a magnetron sputtering process, the glass substrates for the mirrors are first bent as desired typically in a size that would produce two or more mirrors. After the bent glass pieces are washed, they are placed on an appropriate carrier and are coated by magnetron sputtering. Due to curvature of the substrates, the reflective coatings that are produced have not been precisely uniform. The manufacturing process itself is tedious and time-consuming inasmuch as it requires multiple small glass substrates to be laid by hand upon a carrier that passes through a magnetron sputtering apparatus, and requires each of the resulting individual mirror pieces to be removed by hand by the carrier sheet once the sputtering operation is complete.
To avoid these problems, it would be desirable to first sputter deposit a reflective coating on a flat glass sheet to form a mirror, and then heat-bend and cut the mirror as desired. U.S. Pat. No. 4,826,525 (Chesworth et al.) shows an example for the preparation of a mirror through the use of consecutive chromium and aluminum coatings on glass. In general, when flat glass sheets are coated with a reflecting layer using chromium, for example, as the reflective metal, and then are heat bent at the softening temperature of glass, the coatings may develop defects which may be referred to as pits. The pits appear as visually detectable, small, circular defects having little reflectance. The phenomena of pitting (sometimes referred to as xe2x80x9cpinholingxe2x80x9d) is not fully understood, but it is believed to be a function of stresses developed during the bending operation in one or more of the reflective sputter deposit films forming the reflective layer.
The present invention relates to a heat-formable mirror that is capable of being configured at elevated temperatures into a curved mirror without the occurrence of pitting and without significant changes in reflectance. The heat-formable mirror comprises a substrate, preferably glass, bearing, from the surface of the substrate outwardly, a transparent amorphous layer and a reflective layer. The latter comprises first and second contiguous but different metal films. The first metal film is a reflective film that is positioned nearer the substrate than the second metal film, and the second metal film comprises a protective film that is less reflective than the first metal film. The second metal film may be of niobium, tungsten, tantalum, iron or nickel, niobium being greatly preferred. The reflective metal film, which is preferably aluminum, is of sufficient thickness as to provide the mirror with a reflectivity of at least 50%, and the second metal film is present in a thickness sufficient to protect the first metal film and to avoid significant reduction of reflectivity during heat forming of the mirror. The product as thus described may also have a protective film positioned further from the substrate than the reflective layer, the protective film preferably comprising a dielectric oxide or nitride such as sputterdeposited silicon nitride, sputter deposited aluminum oxide or sputter deposited silicon oxide. Of these, silicon nitride is preferred.
When a heat-formable mirror of the invention is heat formed at a temperature above the temperature at which layers of the reflective coating are deposited, atomic diffusion and/or structural rearrangements can occur between the various sputtered films, changing the reflective properties of the bent mirror product. The heat-formable mirrors of the invention, however, largely and preferably fully retain their important optical mirror properties (low transmissivity, high reflectance) when subjected to heating and bending in this manner, and moreover are free from the pitting phenomenon.
In another embodiment, the invention provides a curved mirror that is produced by providing a heat-formable mirror of the type described above, and subjecting the mirror to bending forces at a temperature at which the substrate is capable of plastic deformation (e.g., the glass transition temperature in the case of glass substrates). The flat mirror is bent at that temperature into a desired curved configuration to produce the curved mirror, the latter being cooled while maintaining its curved configuration. The resulting curved mirror desirably retains at least about 100% of the reflectance and not over about 150% of the transmissivity of the heat-formable flat mirror from which it was made, and is essentially free from observable pitting.
Curved mirrors of the invention desirably display a hemispherical reflectance (as measured using a reflectometer and integrating sphere over the wavelength range of 200 to 2600 nm) of at least 50% and a transmissivity not greater than about 4.0%. xe2x80x9cReflectancexe2x80x9d herein is measured using a reflectometer utilizing a tungsten lamp at a filament temperature of 2854xc2x0 K. at an angle of incidence of 25xc2x0+5xc2x0 utilizing a detector cell approximately duplicating the human eye (CIE standard photopic curve) and an integrating sphere. In addition to exhibiting good optical properties for a mirror product, the film stack should be physically and chemically durable in both the flat and bent states.